What should the distribution system pressure be maintained above?

The distribution system pressure should be maintained above underground hydrologic pressure to ensure that water does not flow back into the distribution system from the surrounding soil or aquifer. This pressure control is essential to prevent contamination and maintains the integrity of the water supply. If the pressure in the distribution system drops below the underground hydrologic pressure, it creates a vacuum effect that can potentially draw contaminants from surrounding areas into the water system. Maintaining a higher pressure also helps in delivering water effectively throughout the distribution network and ensures adequate supply to consumers, even in situations of high demand or during fire flows.

The other options do not adequately address the critical need to prevent backflow or contamination risks inherent in water distribution systems. Atmospheric pressure relates to conditions above ground and does not directly impact water quality in the same way that underground hydrologic pressure does. Surface hydrologic pressure pertains to surface water bodies and has no relevance to the underground pressures impacting water systems. Groundwater pressure is indeed important, but specifically maintaining pressure above underground hydrologic pressure is what directly protects public health and safety in this context.